1. Field of the Invention
The present invention relates generally to magnetic recording media, and more particularly to a method for writing and/or reading magnetic bits onto a magnetic recording media. Still more particularly, the present invention relates to a method for synchronizing a recording head with the bit islands on discrete bit patterned media.
2. Description of the Prior Art
Designers, manufacturers, and users of computing systems require reliable and efficient digital information storage and retrieval equipment. Conventional storage systems, such as magnetic disk drives, are typically used and are well known in the art. As the amount of information that is stored digitally increases, however, users of magnetic recording media need to be able to store larger and larger amounts of data. To meet this demand, designers of magnetic recording media are working to increase the storage capacity of a recording disk, which is a function of the number of closely spaced concentric tracks on the surface of the disk. Designers are also working on techniques to write digital data more efficiently, thereby utilizing the storage capacity of a disk in a more effective manner.
FIG. 1 illustrates a prior art method for writing magnetic bits onto a magnetic media. A write head 100 writes a plurality of magnetic bits 102, 104, 106, 108, 110 along a track 112 as the write head 100 passes over the track. The plurality of magnetic bits 102, 104, 106, 108, 110 are written adjacent to each other, as well as adjacent to the plurality of magnetic bits 114, 116, 118, 120, 122 on track 124. Unfortunately; there are several limitations to this method. First, the width of the write head 100 can vary significantly from disk to disk. In fact, in some disk storage systems, the width of the write head can vary from the width of a track by a factor of 2 to 1.
The width of the write head 100 determines the width of the magnetic bits. A narrow write head may not completely erase the information previously written on the track (e.g. track 112). Similarly a wide write head may write unwanted information on an adjacent track (e.g. track 124). When data is read from the disk, interference between unwanted information and non-erased information can result in a poor signal to noise ratio.
Another problem is the fringe fields created by the write head 100. When the write head 100 is writing magnetic bits on track 124, for example, the fringe fields can erase the magnetic bits stored on adjacent track 112. Furthermore, the fringe fields can cause the written magnetic bits to have curved ends. Magnetic bits are best written with straight, radial edges because it makes it easier for the head to read the bits. Curved ends on the bits are undesirable because they make it more difficult to read the magnetic bits.
Adjacent magnetic bits can also destabilize each other on the same track, creating yet another limitation to this method. Magnetic bits are made up of a number of magnetic grains. Small grains are less stable than larger grains due to thermal agitation. This thermal instability is called the super paramagnetic effect and is exacerbated by the presence of the magnetic fields from adjacent bits. These fields can cause the grains at the edge of a bit to switch, resulting in interbit noise. This interbit noise is sometimes called zigzag noise for longitudinal recording due to the ragged way the transitions between bits form. In extreme cases, an entire bit can be switched causing a loss of data.
Another prior art method for writing magnetic bits is depicted, in FIG. 2. Grooves 200, 202 are formed in the surface of the disk and between the tracks. Grooves 200, 202 define the radial width of the magnetic bits, and the concern over the varying width of the write head 100 is eliminated. But the issue of destabilization survives. The bits in a track can destabilize adjacent bits along the track. For example, magnetic bit 204 can destabilize magnetic bit 206, and magnetic bit 206 can destabilize bits 204 or 208.
Therefore, what is needed is an improved method for writing stable magnetic bits in a magnetic media.
The present invention overcomes the limitations of the prior art by writing the data on discrete islands on the media. These islands may be magnetically dead regions between the tracks and between the bits or by depression in the media surface between the tracks and between the bits. The later depressions would be deep enough so that the write head could not magnetize the media in the depressions and the read head could not sense the magnetic fields from the depressions. The present invention provides a method for initially synchronizing a recording head with the bit islands on discrete bit patterned media.
The write head and/or read head is synchronized to the discrete bit pattern by determining the correct clock cycle (phase) and clock count within the clock cycle. The delay will slowly vary as a function of temperature, power supply voltage, disk velocity and vibration disturbances. The present invention also provides a method for continuously (or periodically) correcting the changes in effective delay between the clock and the features on the media to compensate for the system parameter variations.
In one embodiment of the invention, the clock from a phase locked oscillator (PLO) will be used as the write clock source. The PLO is locked onto the servo sectors, and starting delay value is selected. Using this starting delay value, at least one pattern is written on the disk. The at least one pattern is then read and the point at which at least one transition occurs is noted. Using at least one transition, a delay correction value is determined and the programmable delay adjusted so each bit in the pattern is written at a desired location on each island. The delay correction value should adjust the delay of the write head so that the pattern is written at appropriate locations on the islands and no transitions occur when the pattern is read.
In an alternative embodiment, the starting delay value is varied by small increments after every servo sector such that one full PLO clock cycle is divided into many parts. A one (1) followed by a zero (0) pattern is written repeated in the data wedges between servo sectors. The disk is then read and the clock phase at which the bit slips is noted. A delay correction value is then determined and the delay adjusted so that the write head writes magnetic bits at appropriate locations on the islands (blocks 812 and 814).
In another alternative embodiment, the read head is separated from the write head. A PLO field is read and the PLO is locked onto the media. The write head then writes in a sync field using a low voltage write current. This creates a coupling between the read and write heads. The MR element is then sensed, and a phase delay is determined. A phase error signal is determined by subtracting phase delay P from an initial phase delay Po. The phase error signal is used to adjust the programmable delay until Po-P equals zero.
The present invention also includes an exemplary method for locating an initial magnetic bit prior to writing magnetic bits onto the patterned media. In this embodiment, the initial bit is the first island in the data wedge immediately following a servo sector, although the method can be utilized to begin writing bits at any desired island. A pattern is written on the disk. In one embodiment, the pattern is comprised of a combination of ones (1) and zeros, one example being 00010111. With each successive pattern writing, the pattern itself is sequentially shifted left or right. For example, if the pattern 00010111 is written in the first sector, then the pattern 10001011 (or 00101110) is written in the second sector. In the third sector the pattern 11000101 (or 01011100) is written, and so on. In this embodiment, the pattern is read in order to determine where you get a transition in the first bit of a data wedge.
In another embodiment, the pattern is comprised of one or more zeros followed by one or more ones, such as 00000011. With each successive pattern writing, the pattern itself is sequentially shifted left or right. In this embodiment, the pattern is read in order to determine where you get a transition in the first bit (or desired bit) of a data wedge.
And in yet another embodiment, the pattern is comprised of a combination of ones and zeros that are decodable. For example, if the pattern 00010111 is written on the disk, then in each sector a portion of the pattern is decoded. For example, the pattern is grouped into 3 bits, beginning with 000. The binary number for this group is zero. In the next sector, moving one bit to the right, the portion of the pattern is 001. The binary number for this group is one. In the next sector the portion is 010 which is two. This continues around the disk. The desired bit location is determined by reading the pattern.
And finally, two exemplary methods for fabricating a patterned magnetic media are disclosed.